1. Field of the Invention
This invention relates to wide-angle zoom lenses, and more particularly to zoom taking lenses for photographic cameras.
2. Description Relative to the Prior Art
It has long been known to make two unit zoom lenses with a negative front focusing and compensating lens unit followed by a positive variator lens unit. Traditionally, in the case of both zoom lens design and fixed focal length lens design, negative front lens element units are used for wide angle applications. This is because the off-axis ray angles are reduced immediately on entry to the lens, reducing the aberration contributions of the lens elements following the negative front lens element unit. In the case of a long focal length lens, there is no such requirement, and because the use of a negative front lens element unit typically results in a longer-than-normal front-vertex-to-image distance, its other advantages have not offset the disadvantages. The use of such two unit zoom lenses has generally been limited to relatively small zoom ranges ratios, usually less than 2.5:1. This is partially because a moving lens element unit having a change of magnification of approximately 3:1 generally has a large change of aberration due to conjugate change, which of course, depends upon the aperture and field.
To increase the zoom range, it has also been known to make the variator unit in the form of a variable triplet type lens unit with variable spacing within that unit, to thereby provide the additional aberration compensation required by the additional change in magnification. This approach, however, further increases the overall length of the lens, making it undesirably long, particularly in the case of relatively large zoom ratios.
U.S. Pat. No. 4,304,466 discloses that a zoom lens with a negative front lens element unit can have a relatively long focal length, i.e. a maximum field angle of less than 45.degree. and a zoom ratio extending beyond 3:1, while at the same time being quite compact, thus taking advantage of the relative simplicity afforded by the negative front lens element unit design while avoiding the long length normally associated with that type of construction. To achieve these objectives, the foregoing patent specifies that the negative front lens element unit is a biconcave negative lens element and that the variator lens unit, which has overall positive power, is divided into first and second sub-units. The division of the variator unit provides for additional aberration compensation, and, if the second sub-unit has significant optical power, to also provide additional magnification change.
While in general it is ideal to equally divide the optical "work" of magnification change among the available lens units, the above identified patent discloses that a long focal length zoom lens of the type having a negative front unit; and an overall positive variator unit, comprising first and second sub-units, can have relatively few lens elements and no aspherical surfaces if the first lens sub-unit provides the majority of the magnification change. This is because that sub-unit must of necessity be of strong optical power, being the positive real image forming lens unit. It thus requires relatively small movement of the sub-units to achieve the desired magnification change, which in turn reduces the change of off-axis aberrations while zooming. The required aberration correction can be thus provided by an all-spherical second lens sub-unit.
For long focal length zoom lenses, the total front vertex-to-image distance must be minimized for ease of use. The foregoing patent discloses that this can be accomplished by use of a relatively strong negative power lens assemblage for the second variator sub-unit. Additionally, this second variator sub-unit is spaced from the first, positive lens sub-unit in order to provide aberration correction for the off-axis imagery, particularly of those aberrations due to changes introduced by zooming. In order to provide sufficient off-axis aberration correction, it is necessary to provide substantial separation between the two sub-units of the variator unit; the result being that the first principal point of the overall variator unit is located in front of its forward vertex, as disclosed in U.S. Pat. No. 4,304,466 which also contributes to the compactness of the lens.
The above-mentioned copending U.S. patent application Ser. No. 076,893 discloses various short focal length zoom lenses of the general type described above but in which aspherical surfaces are used in the variator unit to correct aberrations. In all of the disclosed short focal length lenses the majority of the magnification change during zooming is provided by the negative power variator lens element sub-unit. As in the case of the disclosure of the previously mentioned U.S. Pat. No. 4,304,466, a biconcave negative power element is employed in the first negative power lens unit in all of the examples having negative-positive-negative configuration with the latter two components comprising the variator lens element unit.